Freeze-thaw cycles may cause damage to concrete and other hardened cementitious materials due to freezing and expanding of water. While such hardened materials appear solid they are porous, having small capillaries resulting from the evaporation of water beyond that required for the hydration reaction during the curing of the material. Excess water that is not required for the cement particles to hydrate evaporates, leaving little pores in its place. En-vironmental water can later fill these voids. During freeze-thaw cycles, the water occupying those pores expands and creates tensile forces to the structure leading to cracks, fissures and surface scaling of concrete. Concrete may have 10% water capillaries due to extra water used in the production to give workability to the concrete in the production phase. In order frost damage to occur, the concrete must be almost saturated, meaning capillaries need to be full of water.
Gas-entrainment, in particular air-entrainment, of concrete increases the durability of the hardened concrete in climates subject to freeze-thaw. Furthermore, it increases workability of the concrete while in a plastic state. In air-entrained concrete, air voids are formed in a way that they intersect the capillaries at regular intervals. Preferably the maximum distance from any point in the capillary system to the surface of the nearest air bubble (void spacing factor) is 0.2 mm. When the concrete becomes wet, water will end up to the capillary system by capillary suction and for the most part the air voids remain empty. When the water in the capillaries starts to freeze, the pressure in the capillary system rises. Before the pressure reaches to a level where it could cause cracking, the water is forced into the air voids and the pressure drops.
Good and robust gas entrainment of cementitious materials consists of both the creation of small gas pores during mixing of the material, and also high stability of these pores before curing i.e. setting and hardening. Robust gas-entrainment refers to the possibility to have some variation in the water content in concrete production, without causing any significant variation in the gas pore structure.
Gas entrainment is attained by use of gas-entraining agents or admixtures, in particular air-entraining agents or admixtures (AEAs), added during mixing or included in a premixed cementitious composition. Entrained air is produced during mechanical mixing of the fresh cementitious material containing AEA. The shearing action of mixer blades breaks up the air into a fine system of bubbles and the AEA acts like surfactants and helps to create smaller air bubbles. In short, concrete air-entrainment means introduction of small, sub-millimeter air pores (0.020 to 0.500 mm) into fresh concrete and cement paste (water+cement) during the concrete mixing process (4 to 8 vol % in concrete and 13 to 25 vol % in cement paste). Yield value and viscosity of cement paste also has an effect on the stability of air in the concrete as high yield value and high viscosity stabilizes the air bubbles in concrete. Entrainment of gases other that air can be achieved for example by use of chemically reactive agents that produce gas, in particular hydrogen, when they react with suitable components of the cementitious composition and/or water during mixing.
Surface active AEAs are commonly and widely used in the production of air-entrained concrete to protect it against frost actions. However, with the known AEAs detailed care must be taken to select proper conditions for avoiding large variations in air content and quality of both fresh and hardened cementitious material. Air pores and other gas pores have a tendency to rise due to buoyance resulting to inhomogeneous variation in the gas pore structure in hardened cementitious material. Usually this separation has been taken into account by increasing the total air content. Migration of gas pores towards the surface of the cementitious layer can cause detrimental results especially in applications where the hardened material is cast into a mold for setting and hardening. Often in these applications the final surface which will be exposed to environment is the bottom part of the cast. The bottom part will, as a result of the rising air bubbles, have poorer air pore quality and will be more prone to cracking caused by freeze-thaw cycles.
There are some possible ways to increase air content stability. Fine particles can be added, commercial stabilizers can be used, and a good air pore structure and small enough pores can be produced by choosing a good mix design and production method, if only possible and economically viable. Water content of cementitious material also should be just high enough to produce separate and small air bubbles. All these methods may work, but there efficiency, usability and expenses vary. There is no generally guaranteed way to produce good gas-entrained cementitious material with desired properties.